The present invention relates to an improved vacuum pump.
More particularly, the invention relates to a turbomolecular vacuum pump with a particularly high compression ratio, capable of exhausting at atmospheric pressure.
Turbomolecular pumps are known which comprise pumping stages with plane or bladed rotors, see for instance U.S. Pat. No. 5,238,362 entitled xe2x80x9cTurbomolecular Pumpxe2x80x9d issued Oct. 26, 1994.
Conventional turbomolecular pumps have rather limited operation ranges. They cannot reach a pressure difference between the inlet and outlet ducts to allow exhaust at atmospheric pressure. Though considerable advances have been made in recent years in pump technology, resulting in the development of turbomolecular pumps allowing exhaust at substantially higher pressures, it is still necessary to provide a so-called fore-pump that is coupled with the turbomolecular pump.
Fore-pumps are coupled outside the turbomolecular pump and their interconnection require gas flow ducts. Moreover, the electrical supply is provided by the same control unit as that supplying the turbomolecular pump. The fore-pump pressure makes the pumping system complex and more subject to failures.
A vacuum generating system, comprising a molecular pump coupled with a fore-pump, is disclosed in U.S. Pat. No. 4,797,068 entitled xe2x80x9cVacuum Evacuation Systemxe2x80x9d issued Jan. 10, 1989. According to the teaching that patent, the exhaust port of a molecular rotary pump, comprising a plurality of pumping stages defined by the coupling of a rotor and a stator, is directly connected with a suction duct of a screw pump. The discharge port of the screw pump exhausts at atmospheric pressure.
The system is characterised by a structural complexity. The system needs two separate electric motors, since the pumps are to rotate at very different speeds. Moreover, even if the fore-pump is equipped with a seal assembly arranged to prevent lubricant from entering the pumping chamber, the molecular pump is subjected to pollution in case of failures or poor maintenance.
Ejector or venturi pumps are also known which are actuated by a first, high-pressure fluid and suck a second, low-pressure fluid thereby generating an intermediate pressure level at the outlet. Both the first and the second fluid can indiscriminately be either a liquid or a gas for instance, by feeding the pump with pressurised water, it is possible to suck a gas such as air, thereby generating a low pressure in a closed space and creating a fore-vacuum condition.
Ejector or venturi pumps, of a kind suitable for sucking a gas, generally can work starting from pressures of about 30 millibars.
Therefore, there is a need to provide a turbomolecular pump capable of exhausting at atmospheric pressure.
The present invention provides a turbomolecular vacuum pump comprising, starting to from the inlet port, a first pumping section having pumping stages with bladed rotor discs, a second pumping section having pumping stages with smooth rotor discs, a third pumping section having at least one pumping stage with toothed rotor disc, and a fourth ejector or venturi pumping section.
According to the present invention, optimised progressive pumping stages are provided in the turbomolecular pump, capable of bringing the exhaust pressure of the turbomolecular pump to a level suitable for the operation of an ejector or venturi pump, typically 30 mbar.
According to the present invention, the turbomolecular pump is capable of exhausting at a pressure of about 100 mbar already at the third stage.
By using a vacuum pump made in accordance with the invention, in particular with a third pumping stage having a rotor disc with straight teeth, an energy saving can be achieved.
Indeed, at the exhaust pressure of 30 mbars it has been experienced that the pump having a pumping stage with toothed rotor has lower electric current absorption than a pump not equipped with a stage with toothed rotor disc.
The vacuum pump according to the present invention can be used in all applications where a high vacuum condition is required in particularly clean environments, such as for instance in semiconductor working processes.